Why isn't the summer solstice the hottest day of the year? Since the sun is directly overhead, I would assume that it is. I haven't been able to find an answer even though I trekked all the way to the library!

Also, Why is it that the seasons are reversed in the Northern Hemisphere and the Southern? I thought it was because the the earth is further away from the sun on it's rotation. I found out that was not correct, but as for the correct answer I don't know. If you could help me figure these out, I would appeciate it!

The summer solistice isn't the hottest day of the year as it takes a while for the atmosphere to warm up. It's similar to the fact that the hottest time of the day is mid afternoon even though the Sun is the highest at mid-day. This can also be explained by the following analogy:

Imagine water flowing into a sink through a faucet, and draining through the hole in the sink. Now, let the water flow into the sink faster than it drains out. In addition, let the faucet be turned slowly so that the rate of water flowing into the sink slowly increases. Clearly, the height of water in the sink will slowly increase. Now, turn the faucet in the opposite direction slowly, reducing the rate of flow of water into the sink. Now, even though the rate of inflow is decreasing, the height of water in the sink will still increase for some time since the inflow rate is greater than the outflow rate. The height of water will decrease only when the inflow has decreased below the rate at which water is draining through the hole.

In a similar manner, in the Northern Hemisphere, the amount of heat received by the Earth from the Sun is increasing slowly towards summer solstice. As one goes towards summer, the heat received during the day is greater than the heat radiated during the night and so the average temperature slowly increases. The heat input is a maximum at solstice and decreases after solstice, but the rate of heat input is still greater then the rate of heat dissipation. Hence, the average temperature keeps increasing even after solstice, and it is only later during the year that the average temperature starts decreasing.

The reason we have seasons is that the Earth's axis is slighlty tilted so that in December the Northern Hemisphere is tilted away from the sun and the Southern Hemisphere is tilted towards the Sun. As a result, it is winter in the Northern Hemisphere and summer in the Southern Hemisphere. Similarly in June the Southern Hemisphere is tilted away from the Sun and experiences winter, while the Northern Hemisphere is tilted towards the Sun and experiences summer. It might help if you draw a picture of this. Put the sun in the middle, on one side draw the Earth with a line through the poles tilted away from the sun. This would be in December. Six months later the same tilt on the other side of the sun puts the Northern Hemisphere closer to the sun, so we have summer.

Actually the Earths orbit isn't quite circular. In January we are very slightly closer to the Sun than in June; so one would think that the Northern Hemisphere has slightly milder winters and cooler summer than the Southern Hemisphere. However, in reality, summers are hotter in the northern hemisphere. The reason is as follows:

There is more land in the Northern Hemisphere, and more water bodies in the Southern Hemisphere. Now, land has a much lower specific heat capacity than water; in other words, water can hold a lot of heat while land cannot. Hence, land heats up faster and also cools off faster than water. So, during summer, the greater amount of land in the northern hemisphere is heated up quicker, while in the southern hemisphere, the water absorbs a lot of the heat and gets warmer by a much lesser amount. In any case, the result is that northern summers are hotter than the southern summers.

About the Author

Jagadheep built a new receiver for the Arecibo radio telescope that works between 6 and 8 GHz. He studies 6.7 GHz methanol masers in our Galaxy. These masers occur at sites where massive stars are being born. He got his Ph.D from Cornell in January 2007 and was a postdoctoral fellow at the Max Planck Insitute for Radio Astronomy in Germany. After that, he worked at the Institute for Astronomy at the University of Hawaii as the Submillimeter Postdoctoral Fellow. Jagadheep is currently at the Indian Institute of Space Scence and Technology.

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